Various commercial articles such as automotive, marine and aircraft bodies, construction and machine parts and furniture are spray painted for surface protection, coloring and other purposes. The paint, which generally comprises solids as well as solvents and/or binder, is typically applied manually or automatically to the object to be painted in an enclosed area, i.e. a spray booth, in order to avoid contamination of the surface of the object being painted and to confine the paint spray to the interior of the booth for protecting the environment and the health of workers at the site. Generally only a portion of the sprayed paint is transferred to the object being painted. The other portion of the sprayed paint, i.e. the portion that is not transferred to the object being painted, commonly referred to as overspray, is usually swept by a gas stream such as a forced airflow through the booth to a separation unit, where the overspray is removed from the overspray-laden gas stream.
In small spray booths, filters are frequently used to remove paint particles from the gas stream. However, such dry separation units require comparatively intensive maintenance and the spent filters have to be disposed, which increases the costs for operation. In large scale systems with high paints consumption, e.g. paint lines in the automotive industry, thus mainly wet separation units such as for example described in DE 44 01 741 C2 are employed. Such wet separation units typically comprise a funnel-type streaming channel located underneath the grid floor of the working area of the booth. Separation liquid is continuously supplied at the upper rims of the streaming channel and flows as a continuous film under the effect of gravity over the channel walls thereby capturing a portion of the overspray particles from the gas stream, which is conducted downward by the funnel-like structure. Downstream of the funnel-like structure a nozzle is provided, which causes intense mixing of the overspray-laden gas stream with the separation liquid in a turbulent zone thereby removing substantially the remaining portion of the overspray particles from the gas stream. Although adequate cleaning performance may be achieved by such conventional wet separation systems, the energy consumption for maintaining the high velocity gas stream is quite high due to a significant pressure drop at the nozzle. Furthermore, the gas stream takes up large amounts of moisture due to the intense turbulent mixing with an aqueous separation liquid, which is disadvantageous with regards to a desirable circulation of the gas since it necessitates prior energy intensive adjustment of the temperature and moisture content of the gas.
WO 2010/025810 A1 discloses a system and process, which do not rely on turbulent mixing for efficient removal of paint overspray from the gas stream, but are rather based on electrostatic scrubbing. Herein an overspray-laden air stream is conducted e.g. via two spaced downward converging baffle plates arranged in V-shape underneath the paint spray booth to an electrostatic wet deposition device for removal of the overspray particles from the gas stream. This deposition device includes multiple parallel and vertically arranged deposition units spaced from one another, which each comprise two parallel spaced rectangular side sheets whose upper edges are connected via a curved top having a semicircular cross-section with a central groove. A grid electrode topped by wire electrodes is arranged parallel-sided between each pair of deposition units and is connected to a high voltage source to charge the electrodes negatively with respect to the deposition units, which are grounded. Overspray particles entrained in the air stream that is conducted downward in the space between the deposition units are ionized through corona discharge at the wire electrodes. The resulting negatively charged paint particles are subsequently accelerated by the potential gradient in the electric field between the respective grid electrode and the adjacent side sheets of deposition units towards the positively charged sheets. A separation liquid is supplied via the groove at the top of each deposition unit to the vertical side sheets thereof where it forms a continuous film flowing downward, which captures and removes the attracted ionized overspray particles from the air stream. The separation liquid is collected at the bottom of the deposition units for subsequent treatment to remove the paint components and may afterwards be returned to the deposition units. A system, which is a further development of the system disclosed in WO 2010/025810 is commercially available from EISENMANN Anlagenbau GmbH & Co. KG under the name E-Scrub. Due to the absence of pronounced bottlenecks in the flow path, the pressure drop in such system is comparatively low. According to information of the manufacturer, the E-Scrub system enables efficient removal of overspray from air streams of paint booths with up to 95% air recirculation, requiring up to 86% less water and allowing a reduction of energy consumption of up to 75% compared to conventional wet separation units for paint spray booths. Nevertheless, the system as operated with conventional separation liquids known in the art remains prone to paint deposits adherent to the deposition sheets, which locally disturb the wetting of the sheet surface and affect the electrical field, and may ultimately cause arcing. Thus special constructional means such as an oscillating mechanical element causing waves on the flowing film to improve the wash off efficiency and/or relatively frequent maintenance work is required to avoid or remove disturbing paint deposits for stable operation of the process.
The separation liquids known in the art for the use in wet separation units for the removal of paint overspray from a gas stream are generally water-based or oil-based. Water is preferable as carrier liquid for economical and safety reasons, in particular when used in wet separation units that include possible ignition sources such as high voltages. Aqueous separation liquids typically comprise additionally one or more than one water-soluble organic solvent that facilitates take up and dispersion of the overspray particles. For appropriate wetting of the deposition surfaces, the surface tension of the liquid is moreover typically reduced by the use of one or more surfactants, most commonly fatty alcohol alkoxylates. Furthermore, as described e.g. in WO 2010/025810, the aqueous separation liquids may contain a detackifying agent such as silicates, salts of Al, Zn, Fe, Ca or Zr, amines or certain polymers for eliminating the tackiness of the captured overspray particles to prevent their adherence to the deposition sheets and a clogging of lines, pumps and containers in the treatment and circulation system. Additionally a thickener may be employed for increasing the viscosity of the liquid to enable the formation of a continuous coherent film flowing downward on strongly inclined or even vertical deposition structures. Among the thickeners previously employed for the formulation of aqueous separation liquids are inter alia high molecular cellulose derivatives such as Natrosol 250 HHR, which has a Brookfield viscosity in 1 wt. % aqueous solution of about 3,400 to 5,500 mPa·s. In accordance with the general understanding, the high molecular weight of these compounds renders them particularly effective as thickener requiring thereby only low amounts for adjusting the desired viscosity level of the separation liquid. There is no disclosure in the prior art about employing cellulose derivatives of low molecular weight in aqueous separation liquids for the removal of paint overspray.
Conventional aqueous separation liquids typically have a viscosity in terms of outflow time as measured according to ISO 2431 with a 4 mm flow cup at 20° C. of more than 40 s. Such viscosity markedly higher than the viscosity of water is considered helpful in obtaining a substantially continuous coherent film flowing on strongly inclined surfaces and provide sufficient cohesion to prevent local dewetting or a detachment of droplets e.g. at local defects or edges on the flow path of the liquid. Due to their substantial viscosity these conventional aqueous separation liquids are used in wet separation units at comparatively low flow rates, typically of about 2 L/(m2·min) or less. However, de-aerating the liquid as well as cost-efficient industrial standard methods for separation of the paint components from the used separation liquid such as centrifugation-based techniques become increasingly difficult to carry out the higher the viscosity of the separation liquid. Moreover, it is difficult to achieve the desirable film uniformity avoiding e.g. the formation of hangers at these viscosity levels.
EP 2 365 037 A1 proposes aqueous separation liquids, which have a significantly lower viscosity comprising at least 49.95 wt. % water, at least one non-ionic surfactant and 5 to 50 wt. % of an organic water-soluble film-supporting component selected from certain monomeric or polymeric polyols. Herein, the viscosity may be adjusted by addition of a high molecular cellulosic thickener such as Tylose 100,000 YP2, which was used throughout the Examples. Such low viscosity aqueous separation liquids are compatible with industrial standard methods for separation of the paint components and enable forming uniform continuous flowing films having a film thickness as low as 50 μm or less on strongly inclined or even vertical deposition structures at conventional flow rates. According to EP 2 365 037 A1 the low film thicknesses are seen as a particular advantage because they correspond to a reduction in the amount of separation liquid to be circulated thus saving energy. However, turbulences in the high velocity gas stream and distortions have to be avoided in order to maintain such uniform continuous thin films and prevent a local dewetting of the deposition surfaces. Moreover, for such low film thicknesses there is the risk that some impinging overspray particles, in particular those of large size and/or high impact energy, may penetrate through the liquid film to contact the bare underlying deposition sheet and stick thereto. A lack of cohesion of the liquid prevents on the other hand substantially increasing the film thickness since at the corresponding higher flow rates a detachment of droplets due to inertia occurs at discontinuities on the flow path. Accordingly, it was observed that flowing films of the aqueous separation liquids known from EP 2 365 037 A1, which may be uniform, smooth and continuous on planar inclined deposition sheets, tend to become non-uniform in thickness, to tear and/or to form droplets at edges, distinctly curved elements and local structural defects on the flow path. The presence of edges and curved elements of the structural parts in contact with the separation liquid is common in the design of wet separation units due to requirements such as confining and/or directing the gas stream or effective application of the separation liquid to the deposition structures from a liquid reservoir. With the aqueous separation liquids known from the prior art there may thus occur insufficiently wetted or partly non-wetted areas on the deposition sheets, so that impinging paint particles from the gas stream can adhere to these surfaces leading to possible fouling of the surface, which makes it virtually impossible to retain a continuous film on these surfaces, necessitating maintenance work for removal of the adherent fouling paint. These detrimental effects are even more pronounced in wet electrostatic separation units since any paint deposits as well as films with significantly non-uniform thickness e.g. due to the formation of hangers will significantly affect the electrical field. Furthermore, droplets that detach from the flowing film of the separation liquid e.g. at edges can cause arcing in the high voltage electrical field. It would be desirable to avoid these disadvantageous effects for the sake of a more stable, reliable and maintenance free operation of wet separation units.
Moreover, an efficient removal of the captured paint components from the separation liquids of the prior art typically requires at least in the case of water-based paints the use of flocculants. These costly auxiliary agents are consumed upon paint removal and thus need to be replenished, if the separation liquid is circulated. Moreover, the use of flocculants is associated with the risk of an undesirable removal of functional components such as detackying agent or surfactants from the aqueous separation liquid together with the paint causing a need to control and, if required, adjust the composition of the separation liquid for the sake of a stable operational performance, when recycling the liquid.
Additionally, many aqueous separation liquids for the removal of paint overspray from a gas stream known from the prior art exhibit an undesirable inherent tendency to foaming. The suppression of foam formation thus regularly requires the addition of defoaming agents, which may however interact with other components of the separation liquid in an adverse manner affecting e.g. the film formation and increase the costs of the process. It would therefore be an advantage, if the separation liquid is less prone to foaming or ideally causes no foaming at all.
Consequently, the object of the present invention is to provide an aqueous separation liquid that exhibits enhanced flow characteristics enabling to form and maintain coherent, substantially uniform, continuously flowing films with a smooth or wavy surface structure, without the formation of hangers, on strongly inclined and/or vertical deposition structures even when exposed to a high velocity gas stream loaded with impinging particles and when edges, distinctly curved elements and/or local defects are within the flow path, avoiding in particular tearing of the film and/or droplet formation thereat. Such deposition liquid should be suitable for use in electrostatic wet separation units, allow effectively capturing paint overspray from different sources such as water-based paint compositions as well as solvent-based paint compositions without paint deposits adhering to the deposition structures and enable facile removal of the captured paint components from the separation liquid by established industrial methods, preferably without the need of flocculants. Preferably the separation liquid should have little or no inherent propensity to foaming and allow facile degassing.
Another object of the present invention is to provide an efficient process for removing paint overspray from a paint spray booth having a low energy consumption and enabling a high recycling ratio of all carrier streams used in the process, that allows a stable, safe and reliable operation with no or minimum maintenance for a prolonged period.